The present invention relates to a communication technique for RFID (Radio Frequency Identification) systems and, particularly, to a technique that is effective for improving the quality of communication for transferring data from/to an ID tag (identification tag).
RFID is disseminating as an automatic identification technique in which data can be transferred by wireless communication. An RFID system comprises an ID tag capable of storing information and a read/writer that performs reading and writing information from/to the ID tag. An ID tag comprises a semiconductor integrated circuit device such as, e.g., a non-contact IC chip, and an antenna.
The ID tag receives radio waves output from the read/writer and a rectifying circuit in the ID tag generates an internal supply voltage VDD by rectifying and stabilizing the radio waves received by the antenna. This internal supply voltage VDD is supplied to internal circuits (such as a logical circuit) in the ID tag.
Communication between the ID tag and the reader/writer is performed by modulating radio waves output from the read/writer. Communication from the read/writer to the ID tag is performed such that the radio waves output from the read/writer is ASK (Amplitude Shift Keying) modulated and demodulated in the tag.
A way of communication from the ID tag to the read/writer using a back scattering method is known. This back scattering method is accomplished by changing the input impedance of the ID tag. The input impedance change results in a change in the reflectivity coefficient of the ID tag, which in turn changes the power of the radio waves being reflected to the read/writer.
One known implementation of the back scattering method is such that a switch for back scattering, which comprises, for example, a MOS transistor, is inserted between the antenna terminals of the ID tag (in a path for supplying current to internal circuits).
By turning this switch for back scattering on (to conduct current), which changes the impedance between the antenna terminals of the ID tag and causes an impedance mismatch, a back scattering communication is achieved.
As for this type of communication technique for ID tags by using the back scattering method, an ID tag including a diode switch provided within a rectifier to control back scattering is known (for example, refer to Patent Document 1).
[Patent Document 1]
    Japanese Unexamined Patent Publication No. 2004-54515